1. Field of the Invention
The present invention relates to an apparatus for detecting a direction of visual axis of an eye of an observer, and more particularly to an apparatus which can effectively obtain data for calculating the visual axis The present apparatus is applicable, for example, to a single lens reflex type camera or a video camera.
2. Description of the Related Art
An apparatus for detecting a visual axis has been developed as a means for studying a physiological function of an eye or checking an effect of an advertisement, and it has recently been proposed to use it as a means for inputting a photographing condition of a camera to a controller. Japanese Laid-Open Patent Application No. 1-274736 discloses a single lens reflex camera having a visual axis detector equipped therein
An example of a prior art visual axis detector is shown in FIG. 8 to explain problems in the prior art.
A light beam is irradiated to an eyeball of an observer (photographer) to form a first Purkinje image (cornea reflected image) based on a reflected light from the eyeball and a front eye part image on an image sensor plane in order to detect a visual axis of the eyeball by using positional coordinates of images on the image sensor plane.
Numeral 81 denotes a microprocessing unit (MPU) which carries out various arithmetic operations such as the calculation of the visual axis by using the positional information of the first Purkinje image and the front eye part. Numeral 82 denotes a memory and numeral 83 denotes an interface circuit having an A/D conversion function. Numeral 87 denotes a light projection means which projects an infrared light, which is not visually detectable by to an observer, emitted by an infrared light emitting diode 87a to an eyeball (not shown) of the observer through a projection lens 87b. Numeral 85 denotes a light emission control circuit which controls the light emission of the infrared light emitting diode 87a. Numeral 86 denotes a position sensor which detects a vertical/horizontal position of a camera when the visual axis detector is mounted on the camera.
Numeral 84 denotes detection means having an image sensor 84a, a driver 84b and a lens 84c. It focuses a first Purkinje image based on a reflected light from the eyeball and a front eye part image on a plane of the image sensor 84a through the lens 84c.
A method for detecting a visual axis of an eye has been proposed in Japanese Laid-Open Patent Application No. 2-209125 or Japanese Laid-Open Patent Application No. 2-264632. In the prior art, the visual axis is detected by using information corresponding to two positions, namely the position of first Purkinje image (corneal reflect image of the light source) and the center of the pupil calculated by plural portions surrounding the pupil.
The infrared ray is irradiated to the eyeball of the observer from the light projection means and a position at which a virtual image of the infrared light emitting diode 87a created by the reflection by the front part of the cornea, that is, the first Purkinje image, is formed is detected by the image sensor 84a. The position at which the first Purkinje image is formed corresponds to the position of the pupil center when a rotation angle of the eyeball is zero (the visual axis of the eyeball) and the position deviates from the pupil center as the eyeball rotates.
The deviation (distance) between the first Purkinje image and the pupil center is substantially proportional to sine of the rotation angle of the eyeball. Thus, the distance is determined based on the positional information of the first Purkinje image and the pupil center. The rotation angle of the eyeball and the correction of the visual axis, (that is, compensation for an error of the visual axis relative to the optical axis) are calculated to determine the visual axis of the photographer.
An integration time of the image sensor when it senses the light beam is set by taking into consideration various conditions such as a light emission intensity of the infrared light emitting diode, a sensitivity of the image sensor, an S/N ratio, and a usually anticipated external light in photographing. Thus, the image sensor integrates the light beam for the preset integration time period.
As a result, various problems may arise depending on the illumination intensity of the front eye part of the eyeball. For example, if the illumination intensity is low, a contrast (a difference between output values) between the pupil and an iris is small and it is difficult to detect a contour of the pupil. On the other hand, if the illumination intensity is very high, the image sensor saturates and a difference between the output signals of the first Purkinje image and the iris image, which inherently has a difference, is lost, and it is impossible or difficult to detect the first Purkinje image.